This invention relates generally to optical devices such as projection displays and scanners using an acousto-optic light modulator.
It has been known for some time that light may be diffracted from periodic structures such as those created by sound. Acousto-optic devices have been used as modulators, scanners, spectrum analyzers, filters, frequency shifters and isolators.
An acousto-optic modulator includes an acoustic medium such as tellurium dioxide to which a piezoelectric transducer is bonded. An electric signal applied to the piezoelectric transducer is converted to sound waves propagating inside the acoustic medium. The frequency spectrum of the sound waves matches that of the electric excitation. The sound wave pressure pattern creates a travelling wave of rarefaction and compression which in turn causes an analogous perturbation of the refractive index of the acoustic medium.
The acousto-optic modulator may be thought of a phase grating with an effective grating line separation equal to the wavelength of the sound in the acoustic medium. A phase grating splits incident light into various orders. The angle between neighboring orders equals the ratio of the wavelength of the incident light inside the acoustic medium to the wavelength of sound in the acoustic medium and increases by a factor proportional to the refractive index of the medium.
In the process of interaction between phonons and photons there is conservation of energy and conservation of momentum. For a sufficiently wide acoustic transducer, only one diffracted order may be generated. There are prescribed angles of incidence and diffraction in the acoustic medium for plane waves of sound and light to interact such that the directions of incidence and scatter differ by an angle of twice the Bragg angle. This form of diffraction is called Bragg diffraction.
The sign of the Bragg angle is equal to the ratio of the wave number associated with the acoustic wave over twice the wave number of the optical wave. The wave numbers are equal to 2.pi. divided by the wavelength of either the optical or acoustic wave. For any angle of incidence, the acoustic vector will complete an isosceles triangle defining the angle of diffraction. The highest efficiency is obtained when the tip of the vectors lie within a circle as described in the momentum matching diagram. The maximum diffraction efficiency is obtained at angles that satisfy the Bragg condition. The Bragg condition is satisfied by sufficiently wide transducers. The condition at which multiple diffraction orders are obtained is termed the Raman-Nath regime.
Previous acousto-optic displays have used a time sequenced or sequential operation. One row of data from either a transmissive or reflective spatial light modulator is imaged onto the acousto-optic modulator. The modulator deflects the line of data along the vertical dimension of the screen. A synchronously rotating color wheel or a synchronously switched liquid crystal color filter is used to provide time sequential scanning for multi-color displays. Thus, in a first instance of time data corresponding one color is passed through acousto-optic modulator and quickly thereafter data corresponding to the other two color planes are passed through the acousto-optic modulator in succession. If the timed sequence is fast enough, the user does not notice the sequencing effect.
Unfortunately, techniques which require timed sequential display result in relatively inefficient devices. Because only one wavelength of light is used at a time in a timed sequence, the majority of the incident light developed by the light source is wasted. Therefore, these devices have exhibited relatively poor light efficiency.
Thus, there is a continuing need for spatial light modulators using acousto-optic modulators which are capable of demonstrating improved efficiency.